1. Field of the Invention
The present invention relates to an apparatus for supporting substrates in a substrate processing system. More specifically, the invention relates to a modular substrate cassette for storing substrates in a substrate processing system.
2. Background of the Related Art
In the fabrication of electronic devices such as integrated circuits, multiple conducting, semiconducting and dielectric materials are deposited on and removed from substrates to form the electronic devices. Cluster tools which combine numerous substrate processing units in a processing platform are generally used in advanced microelectronics fabrication. A cluster tool generally refers to a modular, multi-chamber, integrated processing system, which typically includes a central substrate handling vacuum chamber and a number of peripheral vacuum processing chambers. A batch of substrates are typically loaded into the cluster tool and undergo a series of one or more process steps. The process steps are usually performed under vacuum conditions in the various process stations without being exposed to ambient conditions. The transfer of the substrates through the processing system is managed by the central substrate handling vacuum chamber, typically referred to as a transfer chamber. Different configurations of cluster tools, such as linear or radial, are available and offer high yields of devices due to lower defect densities.
Cluster tools typically include at least two load lock chambers mounted on the transfer chamber for loading or unloading substrates while maintaining the vacuum conditions in the transfer chamber and the process chambers. Unprocessed substrates are loaded into the system through one load lock chamber while processed substrates are unloaded from the system through the other load lock chamber.
FIG. 1 is a top schematic view of a commercially available substrate processing system, known as an Endura.RTM. platform, available from Applied Materials, Inc. located in Santa Clara, Calif. The platform combines vacuum processing chambers designed to process substrates at low pressure/high vacuum in the range of about 10.sup.-3 to 10.sup.-8 torr. A loading robot 8 using a blade 7 loads and unloads substrates into and from a cassette 10 disposed in each of the load lock chambers 12 and 14. Substrate cassettes are used for transportation, storage, inspection, and other purposes in processing substrates. The face 44 of the cassette 10 is typically aligned in the load lock chamber with the load lock access 17 of the first load lock chamber 12 or the second load lock chamber 14, so that the loading robot 8 with the blade 7 accesses the substrates in the direction of arrow 13. Typically, after loading the substrates, the cassette 10 is rotated and the face 45 of the cassette is aligned with the first slit valve 9 or the second slit valve 11.
A first transfer robot 16 having a blade 18 is located in a buffer chamber 20. The substrates are introduced into and withdrawn from the platform through a first slit valve 9 communicating with first load lock chamber 12 or through a second slit valve 11 communicating with a second load lock chamber 14 in the direction of arrow 15. The first transfer robot moves the substrate 22 between various pre/post processing chambers 24 and 26 surrounding the buffer chamber 20. The buffer chamber 20 and the transfer chamber 32 are connected through two common chambers 28. A second transfer robot 30 is located in a transfer chamber 32 to transfer a substrate 34 between the chambers 28 and 36 surrounding the transfer chamber 32. Chambers 28 may also include a substrate cassette disposed therein to store one or more substrates either on entry or on exit from the transfer chamber and the back end of the platform system where the processing chambers are typically located. A substrate may be processed or cooled/heated in one or more chambers during processing to accomplish fabrication of a desired semiconductor structure on the substrate. A microprocessor controller 38 and associated software are provided to control processing and movement of substrates through the system.
One configuration of a platform aligns the multiple load lock chambers 12 and 14 with the load lock access 17 to allow a single path access between the robot 8 and the cassette 10. However, single path access to robot 16 typically requires the load lock chambers to also be aligned with load lock access 19, typically at an angle to the load lock access 17. In the platform of FIG. 1, the angle is about 20.degree.. As a result of the platform configuration, multi-directional access must be accommodated or a rotating mechanism must be used to rotate the cassettes into the desired alignment. The inclusion of a rotating mechanism increases the complexity of the apparatus and can negatively affect throughput of the system. Therefore, it is desirable to allow the substrate to be retrieved at multiple angles through the cassette independent of cassette rotation. Multi-directional access simplifies the robotic structures, linkages, and programming and allows more flexibility in chamber design.
FIG. 2 is a perspective view of a typical cassette 10 and a substrate 39, shown in phantom lines, disposed therein with the top bracket removed to more clearly view the cassette components. Generally, a substrate 39 is received in a cassette slot 41 formed between two substrate supports 40 and supported in a pair of alignment apertures 42 disposed on opposing substrate supports. The substrate supports 40 are typically either machined into sidewalls of the cassette or are disposed into channels formed into sidewalls of the cassette. In some instances, the substrate supports 40 are manufactured from metal or plastic. Some manufacturers produce injection molded plastic cassettes, some of which allow access from one face only, such as face 44. The metal substrate supports are machined and sometimes coated for wear or corrosion protection. The costs can be several thousand dollars for a substrate cassette due to the difficulty in machining. A preferred material used for the substrate support 40 is quartz or ceramic material because of high temperature dimensional stability, chemical purity, durability, and resistance to corrosive environments under processing conditions. The type of material, however, can create difficulties in the manufacturing process, such as time consuming grinding of the supports 40 to provide the alignment apertures 42 for the substrates. Such manufacturing can encompass hundreds of dollars for each support, totaling many thousands of dollars to about ten thousand dollars for the cassette assembly.
In the semiconductor industry, different semiconductor manufacturers have varying needs in processing. Typically, 25 substrates are supported in a cassette, used to process a "batch". Other manufacturers require 27 substrates in the cassette with two substrates being "dummy" substrates for pre-processing and post-processing procedures. Other manufacturers may order other variations in the quantity of substrates to be supported by the substrate cassette. Some cassettes are used for other purposes such as loading and unloading a batch of cassettes of up to about 100 substrates or more into and from a furnace for other processing, such as annealing. It would be advantageous to readily adapt a standard set of components for a variety of substrate cassette configurations, particularly those providing multi-directional access. Some attempts in providing multi-directional access have been disclosed in U.S. Pat. No. 5,387,067 and U.S. Pat. No. 5,556,248, a continuation of U.S. Pat. No. 5,387,067, and assigned to the assignee of the present invention, where both of the patents are incorporated herein by reference. In those patents, access at an acute angle is disclosed. However, the substrate slots retain the typical and more difficult configuration of substrate slots with the associated machining. Among other aspects, those references do not provide the desired modularity nor separate components for different substrate cassette configurations.
Thus, a need exists for simplifying the construction of a substrate cassette, modularizing the assembly of multi-slot cassettes, and providing multi-directional access into and from the cassette for the transfer of substrates.